1. Field of the Invention
The inventions are related to silicon bipolar RF-power transistors, particularly discrete transistors using high voltage supply for use in cellular base stations, TV-transmitters etc.
2. Description of Related Art
Bipolar transistors for high-frequency power amplification are widely used in output parts of communications system. High-frequency transistors were first fabricated in germanium in late fifties but were soon replaced by silicon bipolar transistors in the beginning of the sixties, and have since then dominated the RF-power area. For cellular radio, bipolar transistors are dominating in the base station output amplifiers, and can deliver great performance up to at least 2 GHz and 100 W output power, with good stability, availability and price. Other technologies of choice for this class of applications are GaAs MESFETs and laterally diffused MOS-transistors (LD-MOS). There is a strong driving force to further improve the existing technology, as well as to explore new types of devices, because of the rapidly expanding telecommunications market. Computer tools presently available are not yet capable to predict detailed behavior or performance in real applications, and performance optimization is made using mainly experimental methods.
Power transistors are especially designed to deliver high output power and high gain. Manufacturing process, device parameters, layouts and package have been carefully tuned for this purpose. The devices need to meet numerous detailed requirements for breakdown voltages, DC gain or transconductance, capacitance, RF gain, ruggedness, noise figure, input/output impedance, distortion etc. The operating frequency range from several hundred MHz into the GHz region. Power transistors operate at large signal levels and high current densities. About 1 W output power is a starting level where special considerations have to be taken into account, and may serve as a loose definition of a power device, compared to a xe2x80x9cnormalxe2x80x9d, IC-type of transistor.
A bipolar transistor is usually designed using only one n-type (i.e. NPN) device on a single die. A collector layer (nxe2x88x92 epi) is epitaxially deposited on an n+ substrate. The base and emitter are formed by diffusion or ion implantation at the top of the epitaxial layer. By varying the doping profiles, it is possible to achieve different frequency and breakdown voltage characteristics. The output power requirements range up to several hundred watts, sometimes even kilowatts, and the high output power is achieved by paralleling many transistor cells on a single die, and paralleling several dies in a package. The packages often have large gold-plated heat sinks to remove heat generated by the chip.
For the DC-data, the BVCEO (collector-emitter breakdown voltage with open base) is the most limiting parameter, traditionally designed to be higher than VCC (24-28 V supply voltage is a common range for this class of devices). A well-known empirical formula for the relationship of the transistor breakdown voltages and the current gain, b or hFE, states:                               BV          CEO                =                              Bv            CBO                                β            n                                              (        1        )            
where BVCEO already has been defined, BVCBO is the collector-base breakdown voltage with open emitter, and n is an empirical constant, usually between 2.5 and 4.5, related to the nature of the BC-junction breakdown. For a given epi doping and device design (constant n), BVCBO will be constant, and then BVCEO and xcex2 are directly correlated: higher xcex2 gives lower BVCEO. n can be improved by different doping profile tricks, to ensure that nature of the BVCBO is as close as possible to the one-dimensional junction case.
To obtain a device capable of high output power, the doping of the collector layer should be selected as high as possible, thus suppressing high current phenomena, such as the Kirk effect. A highly doped collector layer also has the advantage of having a smaller depletion region, which makes it possible to select a thinner epi layer, with less parasitic resistance and better high-frequency performance, without being limited by thickness-limited breakdown. The problem is that increased collector doping inevitably leads to a low BVCBO and thus a low BVCEO, according to equation (1).
To obtain a device capable of high power gain, the xcex2 must not be too low. The power gain Gp can be described by the following relationship:                                           G            p                    ⁡                      (            f            )                          ≈                  β                                    1              +                                                                    β                    2                                    ⁡                                      (                                          f                                              f                        max                                                              )                                                  4                                                                        (        2        )            
where xcex2 is the zero-frequency gain (hFE) and fmax is the maximum oscillation frequency, or the frequency where the power gain is equal to unity. A plot of equation (2), hFE versus Gp, is shown in FIG. 1 for different fmax values at f=1 GHz. From this plot it can be concluded that a high fmax and not too low xcex2 are detrimental for a good RF power gain.
Because of the relations between output power via collector doping, power gain via xcex2 and BVCEO, if a low BVCEO can be accepted, this will lead to significant improvements of the most important parameters for RF power transistors.
Because of this, data sheets may specify BVCER instead of BVCEO. A small resistor is connected between the base and emitter when designing the amplifier, to assure that the base is never fully open. If the resistor is small enough, BVCER will approach BVCES, which is close (slightly lower) to BVCBO. The characteristics for the different collector breakdown voltages are shown in FIG. 2.
As apparent from the previous section, if BVCEO is lower than VCC, an external resistor, which occupies additional space on a circuit board, must be used to assure safe operation of the device. The value is dependent on the size of the device, and an optimal value may be problematic to find, and requires some experience to not destroy the device while finding the value. If, in any way, the resistor disconnects from the circuit, e.g. during evaluations, bad soldering etc., the transistor may be damaged.
By integrating a resistor on the bipolar RF-power transistor semiconductor die, between base and emitter in accordance with the present invention, it will be assured that the conditions to obtain the BVCER always will be fulfilled.
Therefore, integrating the resistor necessary for BVCER into the semiconductor die results in that the use of transistors with an intrinsic low BVCEO is simplified.
A transistor device according to embodiments of the present invention is set forth herein.